Method for manufacture of EMI reducing circuit card apparatus

ABSTRACT

A circuit card comprises a printed wiring board, a plastic faceplate therefor, and a resilient electroconductive sheet metal strip extending the length of such plate and fastened to its rear side. The strip at its opposite ends provides spring contacts electrically coupled by the strip&#39;s middle part. The card is adapted to be inserted in a holding bay with a rectangular front opening of greater width than height and bordered at its top and bottom by designation strips for the cards to be put in such bay. Metallic structural means adjacent the opening provides an a.c. circuit path extending around the opening in a closed loop. When the card is inserted in the bay, the spring contacts engage with metal-surfaced regions on the designation strips to provide an electrical shunt across such loop and, thereby, to reduce electromagnetic interference radiating through such opening from the bay&#39;s interior.

This application is a division of application Ser. No. 352,795 filed May16, 1989, now U.S. Pat. No. 4,991,062.

This invention relates generally to apparatus which constitutes acircuit card or which utilizes circuit cards. More particularly, thisinvention relates to apparatus of such kind adapted to reduceelectromagnetic interference ("EMI") and to related methods.

BACKGROUND OF THE INVENTION

Many electrical systems and equipment are characterized in use by theradiation therefrom of extraneous electromagnetic wave energy generatedas an incident to the operation thereof. Such systems and equipment runthe gamut from, say, power lines (60 cycle hum) to, say, microwave waveovens (microwave energy leakage), and the frequencies of such radiatedextraneous waves are distributed over a correspondingly broad spectrum.The presence in the environment of radiated energy of such sort isgenerally undesirable because it is a form of "noise," and because itmay adversely affect the operation of other electrical or electronicsystems and equipment in the vicinity of the source of its emanation.Accordingly, that kind of emitted electromagnetic wave energy isreferred to as electromagnetic interference or, as abbreviated, "EMI."

With the advent of telecommunications and computing equipment whichemploy digital techniques and clock pulses having a high repetitionfrequency, the problem of EMI emission therefrom has become troublesometo the extent that the Federal Communications Commission has promulgatedregulations providing that the EMI radiation from such equipment mustnot exceed certain prescribed maximum permissible levels. A furthercharacteristic of such equipment is that it employs as plug-incomponents a large number of circuit cards and (as described in thearticle entitled "Modeling Electromagnetic Interference Properties ofPrinted Circuit Boards," by Clayton R. Paul, published on pages 33-50 ofthe January 1989 issue of the "IBM Journal of Research and Development,"the boards of those cards are rich sources of EMI. Consequently, thereis a need for means to reduce the EMI emitted by this equipment andoriginated from the cards therein to the point where such interferenceis less by a safe margin than the maximum permissible level specifiedtherefor by the FCC and, preferably, is far enough below that level toassure that it will not be exceeded in any foreseeable circumstance ofpractical use of the equipment.

In equipment of the kind considered which employs circuit cards ascomponents thereof, the cards are commonly retained in the equipment inbays which are spaces between shelves included in the equipment. Eachsuch bay is bounded at its top and bottom by upper and lower ones ofsuch shelves, at its rear by a backplane, and at horizontally oppositeends of such rear, by vertical sidewalls extending from the rear to thefront of the bay. Each such bay is, moreover, open at its front and hasa front opening in the shape of a rectangle of greater horizontal thenvertical dimension. The one or more bays of such equipment each normallycontain a bank of circuit cards inserted into the bay from its front andretained in the bay such that the cards in the bank are vertical and inside-to-side stacked relation. When such cards are included intelecommunications equipment wherein each card serves a communicationschannel or channels respective thereto, such a bank is called a "channelbank." Here, however, such a bank will be referred to more genericallyas a card bank or circuit card bank.

When the described circuit card apparatus is operating, electromagneticwave energy originally generated by circuit cards in a bay escapes asradiation from that bay into the exterior environment through the bay'srectangular front opening which is bordered around it periphery by aclosed-loop, a.c. electrical circuit path. In those circumstances, thefront opening acts as a slot antenna for such radiation. It is anempirical rule for such a slot antenna that it is a good radiator onlyof electromagnetic waves shorter in wavelength than the larger of thetwo dimensions of the rectangular opening, the slot antenna tending tosuppress radiation of larger wavelength. It follows from such a rulethat, if EMI escaping from the circuit cards into the bay is distributedfairly evenly over the frequency spectrum, a decrease in the size ofsuch larger dimension will produce a decrease in the EMI "wattage"radiated out through such front opening to the exterior.

As another empirical rule, however, it is only a reducing of the largerof the two dimensions of the front opening which will be primarilyeffective in reducing the EMI power radiated through the opening. To putit another way, if the larger dimension is kept constant and the smallerdimension is reduced in size, the radiated EMI power will not berelatively reduced by anywhere near the amount as the reduction inradiated EMI power effected by reducing the larger opening dimension.Consequently, an aperture at the front of the bay which is narrow but ofthe full length of the bay (as, say, an interstitial gap between thehousing surrounding the front of the bay and the edge of a cover closurefor the bay) will, despite its small width, permit an undesirably largeamount of EMI to escape from the bay to the exterior.

U.S. Pat. No. 4,762,966, issued Aug. 9, 1988, in the name of David E.Kosanda, to Rockwell International Corporation, proposes to reduce EMIescaping from a space within a box for containing circuit cards byproviding a cover for the front of such space so that it is closed attop and bottom except for narrow interstitial gaps left between the topand bottom edges of the cover and adjacent portions of the box, and byproviding for such gaps what is referred to in the patent as an "EMIgasket." That gasket comprises for each such gap a plurality ofresilient metallic fingers on the cover and disposed on its edge on theouter side of such gap to be spaced from each other in the gap's lengthand to project forwardly from the cover towards the portion of the boxwhich is on the gap's inner side. When the cover is attached to the box(in some way not disclosed by Kosanda), the forward ends of the fingersare described by the patent as being forced back towards the edge of thecover or panel, during which time electrical contact is maintainedbetween the cover and the box, and the spacing between the variousfingers for the top and bottom gaps "is such that signal frequencies oflower than a given frequency cannot pass through the opening."

The EMI reducing scheme proposed by Kosanda (to the extent it isunderstood from the disclosure of that patent) has, however, thedisadvantages, among others, that the cover contemplated thereby is anadditional item of expense for the circuit card containing apparatus,hides from view the faceplates of the circuit cards inserted in the box(and thus precludes any casual visual inspection of those faceplates andany associated indicating devices), must be opened or removed each timea card is to be added to or removed from the bank thereof in the box,and takes up space at the front of the box.

SUMMARY OF THE INVENTION

The above-described disadvantages of the Kosanda arrangement areobviated according to one aspect of the invention hereof by providingEMI reducing circuit card apparatus in the form of a circuit cardcomprising a printed wiring board, circuitry on such board, and anelectroconductive shunting means at the front of such board andcomprising a pair of spaced contact means and circuit means electricallycoupling such two contact means together. Those two contact means areadapted, upon insertion of the card into a bay with a front openingencircled by a closed-loop a.c. circuit path, to make respectiveengagements with two contact regions adjacent the card respectivelycoupled electrically to two parts of such path spaced from each otheraround the closed-loop thereof so that such engagements result incompletion through such shunting means of a shunt electrical pathextending between such parts and converting such original closed-looppath into a plurality of closed-loop paths. In terms of the EMI radiatedout through the mentioned front opening, the effect of providing suchshunt path is to divide the original slot antenna at such opening intotwo slot antennae which are reduced in size relative to the originalslot antenna in at least one of its dimensions and which accordinglyeffect a reduction in the wattage of the radiated EMI. If such reductionin size is in the larger of the two dimensions of the original antenna,the EMI reduction is substantial, but even if such reduction in size isin the smaller of such two dimensions, some reduction in EMI wattage isrealized. Moreover, the EMI reduction can be further increased byutilizing a plurality of the circuit cards just described to provide forthe original closed-loop path around the bay's front opening a pluralityof shunt paths extending across the original loop to, in effect, dividethe original slot antenna into more than two slot antennae.

While the invention will be described in terms of an exemplaryembodiment in the form of subscriber loop carrier equipment used intelecommunications, it will be appreciated that the invention isapplicable to any electronic equipment in which circuit cards arecomponents thereof, and to circuit cards adapted to be used in any suchequipment.

BRIEF DESCRIPTION OF THE DRAWING

For a better understanding of the invention, reference is made to thefollowing description of representative embodiments thereof and to theaccompanying drawings wherein:

FIG. 1 is a schematic isometric view of a part of subscriber loopcarrier equipment, and of an EMI reducing circuit card included in suchequipment;

FIG. 2 is a schematic front elevation of the FIG. 1 equipment showingtwo bays thereof and a plurality of circuit cards in one of such bays;

FIG. 3 is a front elevation of the circuit card of FIG. 1 and ofportions adjacent such card of the FIG. 1 equipment;

FIG. 4 is a left side elevation of the FIG. 3 circuit card and ofportions of the FIG. 1 equipment adjacent such card, some of suchportions being viewed in cross section as indicated by the arrows 4--4in FIG. 3, and the card being partially inserted into the holding baytherefor in the equipment;

FIG. 5 is a rear elevation in cross section, taken as indicated by thearrows 5--5 in FIG. 4 of the FIG. 3 card.

FIG. 6 is a rear elevation of the resilient clip component of the FIG. 3card;

FIG. 7 is a right side elevation of the faceplate component of the FIG.3 card;

FIG. 8 is an enlarged schematic view in cross section of a mode forfastening the FIG. 6 clip to the FIG. 7 faceplate.

FIG. 9 is an enlarged schematic view in cross section of the results ofthe practice of the fastening method depicted in FIG. 8.

FIG. 10 is a right side elevation of the front end, broken away, of theFIG. 3 card and of regions, shown in cross section, belonging to thedesignation strips of the FIG. 1 equipment and making contacts with theresilient ends of the FIG. 6 clip of the FIG. 3 card; and

FIGS. 11-13 are schematic electrical diagrams showing the effect of thecontacts depicted in FIG. 10 on a circuit path at the front of the FIG.1 equipment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

Referring now to FIGS. 1 and 2, the reference numeral 20 designatescircuit card apparatus in the form of subscriber loop carrier equipment20 comprising vertically spaced metallic shelves 21, 22, 23 within ahousing 24 provided by a rear backplane 25 and sheet metal side walls26, 27 extending on either side of such shelves from the backplane tothe front of the equipment. Housing 24 is grounded. The horizontalshelves 21-23 and the vertical elements 26-27 provide two bays 31 and 32of which the lower bay 31 is defined between shelves 21 and 22 whileupper bay 32 is defined between shelves 22 and 23. Subscriber loopcarrier equipment such as unit 20 commonly consists of five of suchbays. For economy of drawing, however, only two of them are shown.

Bay 31 is a space bounded at its bottom and top by the top and bottomsurfaces of, respectively, shelves 21 and 22, at its rear by a portionof backplane 25, and at its sides by portions of the sidewalls 26, 27.Bay 32 is similarly bounded by shelves 22 and 23 and by other portionsof such backplane and sidewalls.

The bays 31 and 32 are open at the front of equipment 20 and haverespective rectangular front openings 35 and 36, each having ahorizontal dimension greater than its vertical dimension. The functionof the bays 31 and 32 is to provide receptacles for banks of circuitcards. One such card 60 is shown in FIG. 1 as partially inserted intobay 31, FIG. 2 depicts bay 31 as holding card 60 and an additional card60', while bay 32 is shown as being empty.

The shelves 21, 22 and 23, between which bays 31 and 32 are defined, arerectangular members which, for convenience, are shown as being solid,but which are ordinarily in the nature of perforated gratings. Theseshelves are attached to the housing 24 of equipment 20 by tap screws 40passing at the rear and front of sidewalls 26 and 27 through those wallsinto the vertical sides of such shelves. Since screws 40 are tap screws,they provide adjacent the front openings of bays 31 and 32 a gap-freeelectrical interconnection from the metal of the sidewalls, through themetal of the screws, to the metal of the shelves.

The shelves 21, 22 and 23 are formed to have paired columns 42 of ridges43 vertically projecting from the top and bottom surfaces of each shelfand spaced in each such column in the front to rear direction of theshelf. The two columns of ridges in each pair are transversely spacedfrom each other to define between them a guideway 44, extending in theshelf's front to rear direction, for guiding and holding in place thetop or bottom edge of a particular circuit card inserted into theassociated bay. An exemplary pair of columns 42 of such ridges 43 with aguideway 44 therebetween is shown in FIG. 1 on the upper surface ofshelf 22. The right hand one of a similar pair of columns 42' of ridges43' is shown in the same figure on the upper surface of shelf 21, withthe card 60 being partly inserted into the guideway 44' defined betweenthose two columns.

Mounted by the shelves 21, 22, 23 are three corresponding sheet metaldesignation strips 51, 52, 53 (FIG. 2) attached to the front sides oftheir respective shelves by tap screws 54 passing through the strips andinto the metal of the shelves. In FIG. 2, the ends of strips 51, 52, 53have been broken away to show the underlying shelves. Since screws 54are tap screws, they provide, as before, an uninterrupted electricalconnection from the metal of the strips through the metal of the screwsto the metal of the shelves.

Each of the designation strips has formed in both its top and bottomedges a sequence of notches (FIGS. 2 and 3) spaced from each other inthe transverse length of the strip. Certain of these notches are alignedwith the card guideways 44 defined on the top and bottom surfaces of theshelves. Those notches serve accordingly as card entry notches 55,permitting a particular card 60 to be inserted through that notch into aguideway 44 in one of the bays. There is a card entry notch for both thetop margin and the bottom margin of each card.

The card entry notches 55 may alternate in the length of a strip withother notches 56 which are keyway notches adapted to cooperate with"keys" provided on the circuit cards 60 for equipment 20 to prevent anyparticular circuit card from being fully inserted into a bay of theequipment at any location other than the one at which the card should bereceived.

The outer surfaces of the designation strips 51, 52, 53 are for the mostpart covered with a coating 57 of enamel or other insulating material.At the top and bottom of each such strip, however, there are narrowmarginal bands 58 of the strips' outer surface which are free of enameland are primarily metallic and overlie metallic contact regions 59adjacent each card entry notch 55. If strips 51, 52, 53 are made of anon-oxidizing material such as stainless steel, contact regions 59 canhave bare metal surfaces. Alternatively, the surface areas exposedwithin bands 58 need not be bare metal but, rather, can be covered witha thin film of material which inhibits oxidation of the underlying metal(which may be, say, aluminum) and which may be, for example, thematerial IRIDITE® available from the Witco Corporation, 520 MadisonAvenue, New York, NY. Such thin film provides so little impedance to thepassage of high-frequency electrical current therethrough that,practically speaking, the bands 58 and contact regions 59 can beconsidered to have metallic surface areas.

The circuit card 60 is shown in FIG. 4 (as in FIG. 1) partly insertedinto the bay 31 defined between shelves 21 and 22. Card 60 comprises agenerally rectangular board 70 of insulative material having laterallyspaced top and bottom margins 71 and 72 extending longitudinally from anedge face 73 at the front of the board to the board's rear area 74. Thesurface seen in FIG. 4 of that rear area has thereon an array ofmetallic contact pads 75 adapted when card 60 is fully inserted into bay31 to be received in an edge connector 76 to make electrical contactstherein with corresponding contacts (not shown) in the connector 76.

The board 70 is a printed wiring board ("PWB") having thereon (on itsside seen in FIG. 4) circuitry comprising the mentioned pads 75, printedwiring (not shown), and various discrete electrical components,including an indicating, light-emitting diode 77 and a test jack 78. Thecomponents 77 and 78 are mounted on board 70 to partly project beyondthe front edge face 73 of the board. While the circuitry of the board isdistributed over most of its area on its side seen in FIG. 4, the boardat its top and bottom is characterized on its surface on both of itssides by narrow longitudinal border strips 79, 80, whose innerboundaries are marked by dot-dash lines 81, 82. Within those borderstrips 79, 80, the board is free of any projecting circuitry on itssurface. The purpose of having such border strips is to permit the boardto fit within the entry notches 55 and guideways 44 provided therefor sothat the board can be fully inserted into bay 31.

At the front of board 70 is a laterally (i.e., vertically) elongated,generally rectangular, insulative faceplate 90 which is relativelynarrow in its transverse (i.e., horizontal) dimension, and which isdisposed relative to board 70 such that the centerplanes of the twovertical elements 70 and 90 are at right angles to each other. As shown,the faceplate 90 laterally extends between bottom and top end portions91 and 92 thereof which are laterally outwards of the top and bottommargins 71 and 72 of the board 70.

Faceplate 90 is composed of synthetic resinous material and is moldedfrom such material so as to have various parts which are integral withthe plate, and of which some will now be described.

On its back side, the plate 90 has a pair of plastic bosses 93, 94projecting rearwardly from the main body of the plate. Metal screws 95,96 pass from the far side (not seen in FIG. 4) of board 70 through holestherein and then into the bosses 93, 94 to fasten plate 90 to and infixed relation with board 70.

Disposed above and in transversely displaced relation with boss 93 is acolumnar plastic key 97 projecting rearwardly from the main part ofplate 90. Key 97 is adapted to fit into the particular keyway 56 whichis associated in designation strip 52 with the entry notch 55 at theparticular location in the transverse length of bay 31 at which theparticular card 60 which is shown is to be inserted into that bay. Key97 is so shaped and sized and positioned on faceplate 90 that the keywill not fit into any other of the keyways 56 formed on strip 51. Thekey 97 thus prevents inadvertent full insertion of card 60 into bay 31at a place where the card does not belong.

The plate 90 has formed therein by its molding a small aperture 101 anda large aperture 102 extending through the plate from rear to front andin which are respectively received the LED 77 and the test jack 78. LED77 does not pass all the way through the plate but can be viewed fromoutside the plate at its front to determine if the LED is "on" or "off."Test jack 78 passes all the way through plate 90 to project beyond it.

Plate 90 has formed in its lower right hand outside corner (FIG. 3) alaterally elongated indentation 105 providing for passage through theplate of parts of a plastic latch handle 106 pivotally mounted (by ahollow rivet 108) on board 70 at its lower front corner (FIG. 4). Latchhandle 106 is shown in FIG. 4 as rotated fully counterclockwise to itslatching position at which an integral part thereof constituting aresiliently deflectable pawl arm 109 is inserted into a recess 110formed in the front end of board 70 to engage in that recess with a partof that board constituting a tooth 111 projecting up between that recessand the front edge 73 of the board and holding the latch handle inlatching position so long as the tooth is engaged by the pawl.

Assume that, initially, card 60 is partly retracted from bay 1 as shownin FIG. 4 and assume further, and solely for purposes of explanation,that elements 109 and 111 are engaged as above described to hold latchhandle 106 in latching position. There is of course no need for suchelements 109 and 111 to be so engaged when card 60 is so retracted frombay 31. When, however, such is the case, a finger 115 at the bottom ofthe latch handle 106 extends vertically down to be horizontally oppositethe designation strip 51 and thus block further entry of card 60 intobay 31.

In order to fully insert card 60 into bay 31, the back of pawl 109 ispressed down to disengage its front from tooth 111. Latch handle 106 isthen rotated clockwise to raise the finger 115 at the bottom of thelatch above the top of designation strip 51, and the card is then pushedinto the bay to cause the forward part of the finger to pass by thestrip. Latch handle 106 is then rotated counterclockwise to producereengagement of pawl 109 and tooth 111. In the course of suchcounterclockwise rotation of the handle, the forward part of finger 115passes through an aperture 116 in shelf 21, and the finger's inner sidethen hooks around a bar section 117 provided by the shelf so that thereengagement of elements 109 and 111 causes the card 60 to be locked inbay 31 in fixed position relative to the shelves 21, 22 and thedesignation strips 51, 52. The card may thereafter be removed from thebay by disengaging pawl 109 from tooth 111 and rotating latch 106counterclockwise as before described, and then pulling the card out.

Considering now the EMI reducing features of the described equipment,the card 60 has thereon an electroconductive shunting means disposed atthe front of board 70 and the rear of faceplate 90 and comprising a pairof spaced, resiliently deflectable contact means and circuit meansbridging such pair of means and electrically coupling them together.That shunting means may be conveniently provided by an elongated sheetmetal strip 120 (FIG. 6), say, 5-10 mils thick, constructed of ametallic material which is resilient and a good conductor ofelectricity, and which material may be, for example, but withoutlimitation, stainless steel, phosphor bronze or beryllium copper. Strip120 has bottom and top end portions 121 and 122 which serve aselectrical spring contacts, and, adjacent the inner ends of theseportions, the strip has holes 123, 124 therethrough. Those holes arelaterally spaced apart by the same distance as separates a pair oflaterally spaced plastic studs 125, 126 projecting outwardly andrearwardly (FIG. 7) from the main body of faceplate 90.

The strip 120 is fastened to the back side of plate by heat staking in amanner as follows. Studs 125, 126 are passed through the holes 123, 124in strip 120, and the strip is advanced towards the back side of plate90 until it lies flat against that side with the free ends of the studsprojecting outwards of the outside of the strip. An ultrasonic horn 130(FIG. 8) is then applied forcibly to the projecting free end of stud 125to melt its plastic material by ultrasonic energy, and to convert thatfree end into an expanded locking head 131 (FIG. 8) which rivets theregion of strip 120 around hole 123 to the back of the plastic plate 90.Similar application (not shown) of horn 130 to the free end of stud 126converts that free end into an expanded head 132 similarly riveting theregion of strip 120 around hole 124 to the faceplate.

The electrical contacts provided by strip 120 are in the form ofresilient metal blades having respective anchored inner ends 141, 142adjacent the locking heads 131, 132, and respective free ends 143, 144disposed laterally outwards of the inner boundaries 81, 82 of borderstrips 79, 80 on PWB 70. As shown, they may also lie laterally outwardsof the outer margins 71, 72 of those strips. Those contacts 121, 122 aredisposed so that when in their resiliently unstressed condition, theyslant outwardly and rearwardly from their anchored ends 141, 142 totheir free ends 143, 144, which latter ends are, accordingly,longitudinally displaced rearwardly from the back surface of thefaceplate 90. The end portions 91 and 92 of the faceplate extend in thelateral dimension outwards of the free ends 143, 144 of the contacts121, 122. That faceplate, therefore, provides a backing for thosecontacts over the full length of each.

The contacts 121 and 122 are electrically coupled together by circuitmeans in the form of a flat segment 150 of strip 120, such segment 150extending from one to two other of the anchored ends 141, 142 of thosecontacts. As best shown in FIG. 6, segment 150 has an irregular shapeand, as will be evident from a comparison of FIGS. 5 and 6, the reasonwhy it has such shape is to permit the segment to stretch between thecontacts 91 and 92 in proximate relation to the back side of faceplate90 while concurrently providing for avoidance by the segment of key 97and the bosses 93, 94 and of the rear openings of the faceplateapertures 101 and 102, and for avoidance thereby of blockage by thesegment of passage into such apertures 101 and 102 of, respectively, theLED 77 and the test jack 78, both mounted on the PWB 70.

The circuit card 60' has circuitry on its PWB which differs in layoutand composition from the circuitry on the board 70 of card 60.Otherwise, the card 60' is similar in structural makeup to card 60.Thus, the card 60' includes an electroconductive strip alike in size,shape and mounting thereof to strip 120 on card 60, and the card 60'also includes an indicating LED and a test jack alike to elements 77, 78on card 60 and received like those elements in apertures in theinsulative faceplate of card 60'.

USE AND OPERATION OF EMBODIMENT

Card 60 is used as follows to provide EMI reduction. In the course ofinsertion of the card into bay 31, the contacts 121, 122 of strip 120are at first disengaged from any part of the extent of the metal housing24 for that bay. As, however, such insertion proceeds by advancement ofthe card into the bay, the free ends 143, 144 of such contacts makerespective initial contacts (FIG. 10) with the lower and upperdesignation strips 51 and 52 on that housing (FIG. 2) and, moreparticularly, with the metallic surfaced contact regions 59 provided onsuch strips (FIG. 3) adjacent to the entry notches 55 on such strips inwhich the top and bottom border strips 79, 80 of the PWB 70 arereceived. Because of such engagements between such contact regions 59 onthe strip and the free ends of the contacts 121, 122 on the card, as theadvancement of the card into the bay continues, such contacts are bothresiliently deflected rearwardly so that the free ends 143, 144 of thecontacts and the end portions 92, 92 of the faceplate 90 undergorelative movement towards each other to result in a progressive decreasein the spacing thereof. That deflection of the contacts 121, 122 (andthe consequent stressing of these contacts) renders the engagementsbetween them and the contact regions 59 on strips 51 and 52 a pressureengagement under yieldable force producing a firm and reliableelectrical contact between the contacts on the card and such regions.Each such deflection of the contacts 121, 122 produces a minute wipingaction which keeps the contacts clean.

When the insertion of the card 60 into the bay is completed (as earlierdescribed) by manipulating latch handle 106 to hook its finger 115around bar section 117 and to engage its pawl 109 with the tooth 111 onboard 70 (FIG. 3), the resultant locking of the card in full forwardposition relative to the housing 24 serves to maintain these forcibleengagements between the contacts 121, 122 on the card and the mentionedcontact regions on the strips 51, 52 for so long as the card 60 remainsso locked in position relative to equipment 20. When the card is in thatlatched position, the contacts 121, 122 therein have maximum deflectionbut still have some spacing from the end portions 91, 92 of thefaceplate. Depending, however, on the interaction of latch handle 106with the housing 117 and on other factors, the equipment 20 and card 60can be designed so that the contacts 121, 122 will be deflected all theway back into flush contact with the faceplate's end portions 91, 92which can in themselves be somewhat resiliently deflectable so as to beslightly bent rightwardly (FIG. 4) relative to the center of faceplate90 as a result of locking of card 60 into place by manipulation of latchhandle 106.

When the mentioned contact regions 59 on designation strips 51 and 52are forcibly engaged as described by the contacts 121, 122, the strip120 on card 60 provides a shunt path for a.c. electrical energy betweenthese contact regions. These two contact regions 59 are in turnelectrically coupled to a path for a.c. electrical energy whichsurrounds the periphery of the front opening 35 for bay 31, and which isschematically represented in FIG. 11 by the dot-dash line 160. The path160 defines a single closed loop represented by the arrow 161. The loopof path 160 can be traced as extending (FIGS. 1, 2 and 3) from thecontact region 59 of strip 51 engaged by contact 121, through that stripand the screws 54 in shelf 21 to the shelf; rightward in shelf 21,through the screws 40, attaching wall 26 to shelf 21, to that wall;upward through wall 26, and through screws 40, attaching that wall toshelf 22, into such shelf; leftward in shelf 22 (with which the contactregions 59 on strip 52 are coupled through screws 54) to and through thescrews 40 on the left hand end of shelf 22; downward in wall 27 to andthrough the screws 40 passing into the left hand end of shelf 21; andthen, through the screws 54 in that shelf into strip 51 and back to theplace of beginning. Note that the contact regions 59 on the designationstrips, those strips themselves and the associated screws 54 can beconsidered either as being in the loop 161 of path 160 or as beingelectrically coupled to separate parts of such loop. Note also that themetallic means comprising the members and elements traversed by path 160are grounded as indicated in FIG. 11.

The diagram of FIG. 11 is relevant to a situation in which the bay 31 ofequipment 20 has therein a full bank of circuit cards, none of which,however, is equipped with an electroconductive shunting means the sameas or equivalent to the strip 120 earlier described. The circuit cardsin that bank in their operation emit into bay 31 electromagnetic waveenergy over a spectrum of wavelengths in circumstances in which there isonly one area of front opening 35 encircled by a loop of an a.c. circuitpath, namely, the full area of that opening encircled by loop 161. Allof opening 35 acts as a single slot antenna 175 for radiating EMIconstituted of such electromagnetic wave energy generated in bay 31. Asearlier described, it is an empirical rule for such a slot antenna thatit is a good radiator only of electromagnetic waves shorter inwavelength than the larger of the two dimensions of the slot, suchlarger dimension in the case of front opening 30 being its horizontaldimension. In the situation depicted in FIG. 11 there is no EMIreduction according to the invention hereof.

The FIG. 12 diagram represents circumstances which are changed ascompared to those portrayed by FIG. 11 to the extent that one of thebank of circuit cards filling bay 31 and lacking electroconductive shuntstrips 120 (or the like) has been replaced by the described circuit card60 equipped with such a strip. As shown in FIG. 12, card 60 is insertedinto the bay at a horizontal location about one-third of the way fromthe left hand end to the right hand end of opening 35. When card 60 isso inserted, the contacts 121, 122 on that card engage with two contactregions 59 on, respectively, the lower and upper designation strips 51,52 to complete through the strip 120 on the card a shunt a.c. currentpath 170 between these two separate contact regions 59 and, thus,between two separate parts of the a.c. circuit path 160 extending aroundopening 35. The effect of that shunting of path 160 by path 170 is todivide the original single closed loop 161 into two loops 162 and 163and, correspondingly, to divide the original slot antenna 175 into twoslot antennae 176 and 177. In each of those two antennae, the larger ofits two dimension is its horizontal dimension, but that dimension foreach of slot antennae 176, 177 is smaller than the largest (orhorizontal dimension) of slot antenna 175. From the empirical rule,therefore, that a slot antenna is a good radiator only ofelectromagnetic wave energy of wavelengths shorter than the antenna'slarger dimension, it follows that the configuration shown in FIG. 12will have the effect on the electromagnetic wave energy, generated inbay 31 over a spectrum of wavelengths, of substantially attenuatingradiation through opening 35 of waves of relatively longer wavelengthwhich would not be so attenuated by the FIG. 11 configuration. Hence,EMI reduction is realized in going from the FIG. 11 to the FIG. 12configurations by the insertion, as described, in the bank of circuitcards in bay 31 of just one card 60 having on it a strip 120 providing ashunt path across the a.c. circuit path encircling the front opening ofthe bay.

In the FIG. 12 configuration, as between the slot antennae 177, 176 ofsmaller and larger horizontal dimension, respectively, the antenna 176is controlling with respect to the amount of reduction in EMI powerrealized in relation to the FIG. 11 configuration. That is so becausealthough slot antenna 177 tends to suppress radiation therefrom of EMIwaves of wavelength longer than its horizontal dimension, those of suchwaves which are shorter in wavelength than the horizontal dimension ofantenna 177 can and will still escape from bay 31 through the largerslot antenna 176.

From what has been said, it will be evident that if only one circuitcard with an electroconductive shunting strip thereon is used for EMIreduction in a bank of cards in a bay, the optimum position for thatcard is the midway position in the largest dimension of the frontopening of the bay. That is so since locating the card at that midwayposition leads to the greatest reduction in the EMI power radiated. Ithas been found that by the use of only one such card at approximatelythat midway position, it is possible to reduce the radiated EMI powersubstantially.

FIG. 13 is a diagram representing an extension of the above teachings inthat card 60' with an electroconductive shunting strip thereon likestrip 120 has been inserted into the bank of cards in bay 31 (in placeof another card not having such a strip) so as to have the effects ofdividing the loop 162 (FIG. 12) for a.c. energy into two such loops 164,165 (FIG. 13) and of concomitantly converting the slot antenna 176 ofFIG. 12 into two slot antennae 178, 179 (of FIG. 13). For the reasonsdiscussed above, the FIG. 13 configuration yields greater EMI reductionthan does the FIG. 12 configuration.

The increasing, as described, of the EMI reduction by increasing thenumber of slot antennae into which opening 35 is divided by theinsertion into the bay 35 of an increased number of strip-carryingcircuit cards is a technique which, it has been found, can be desirablycarried out to its limit which occurs when every circuit card in thebank of cards in bay 31 has such a strip providing such a shunting pathbetween parts of the path around the opening. The effect, of course, ofhaving every such card so equipped is to divide the area of opening 35into many slot antennae. When that is done, the horizontal dimension ofeach slot antenna becomes less than its vertical dimension so that bythe mentioned rule of thumb, the size of such vertical dimension wouldthen be primarily controlling as to how much EMI reduction could berealized. Hence, there would seem to be no point in having so many slotantennae.

It has been found, however, that when, by the use of cards with shuntingstrips therein to divide the opening 35 into slot antennae of similarshape, these antennae are increased in number beyond that yielding ahorizontal dimension for each such antenna equal to its verticaldimension, a further reduction in EMI is still achievable. Moreover, itis desirable in any case that every circuit card in the bank of cardsinserted into bay 31 be equipped with an electroconductive shuntingstrip (like strip 120) because, if only one or a few cards in the bankeach has such a strip, it becomes a problem to foretell whether or notthe one or few cards will have the proper positioning within the bay todivide the front opening 35 into a configuration of slot antennae whichmaximizes the EMI reduction or comes acceptably close to so doing.

Accordingly, it is preferred that every circuit card inserted into bay31 have thereon an electroconductive shunting means the same as or theequivalent of the strip 120 of card 160.

Consideration will now be given to some quantitative aspects of theeffect on EMI reduction of the configurations represented by FIGS.11-13. Typically, the distance between the upper lower shelves 21bounding bay 31 (and also, the vertical dimension of opening 35 and thedistance between the top and bottom margins of the boards of the circuitcards 1 is about 3.5 inches. The bay 31 is adapted to hold a bank oftwenty four cards having spacings between centers of about threequarters of an inch when the cards are in their side-to-side relationwithin the bank. It follows that the horizontal dimension of opening 35is about eighteen inches.

The EMI emissions which are of troublesomely high value occur for themost part in the frequency range of 40-90 MHz. For that range the EMIradiated under the circumstances represented by FIG. 11 comes within 1.5Db of the maximum level of EMI permitted for such range by FCCregulations. That 1.5 Db "margin of safety" is, however, so small thatthe is a real risk that the margin will be wiped out altogether ifchanges are made to the design of the circuitry on the cards, or becauseof manufacturing variations or variations caused by differences in thesites at which the equipment is located.

In the circumstances represented by FIG. 12 in which the a.c. circuitpath 160 around opening 35 is shunted by a strip on only one circuitcard included in the card bank at a reasonably central location withinthe horizontal dimension of opening 35, the measured margin of safety isincreased to about 5.5 Db for EMI in the 40-90 MHz range. A safetymargin of such magnitude is a comfortable one for most applications. Ifthe technique of adding more shunts to path 160 is carried out to thecircumstances depicted by FIG. 13 and beyond to the limit at which allof the cards in the card bank have strips like strip 120 shunting path160, then such margin of safety increases for the 40-90 MHz EMI range toabout 9-10 Db, which is a good margin to work with indeed.

The above-described embodiments being exemplary only, it is to beunderstood that additions thereto, omissions therefrom, andmodifications thereof can be made without departing from the spirit ofthe invention. Some examples without limitation of what has just beensaid are as follows.

The mentioned electroconductive shunting means comprising the pair ofspaced contact means and the circuit means coupling them together neednot be integral parts of a single member like strip 120. The mentionedcircuit means can be provided by a variety of conductors such as, say,wire means, and, for high-frequency applications, might even be providedby capacitance instead of by a resistance element. If the faceplate isinsulative, the mentioned electroconductive shunting means may beprovided by an elongated deposit of metallic material disposed on therear surface of the faceplate provided that, again, such pair of spaced,resiliently deflectable contact means are provided by having the endportion of the faceplate being suitably resiliently deflectable.

The electroconductive shunting means of the circuit card need not becarried by the faceplate but, rather, can be carried by the PWB of thecard at the front of the PWB. The arrangement just mentioned isparticularly useful where the card has no faceplate, as happens in somecases. The EMI reduction can be effected not only by a circuit cardsupported in guideways in the holding bay by the structure of the PWB ofthe card but also by a device so supported by a simple slide means inthe form of an insertable contact support unit (as, say, withoutrestriction, a plain board or other planar insertable member) receivedin such guideways and provided with such an electroconductive shuntingmeans. Where the equipment with which one or more of the described EMIreducing circuit cards or devices are used does not have any designationstrips, the invention hereof can be put into practice by having thespaced contacts on the card or cards (or device or devices) engage withregions which are on the framing around the front opening of the bayholding the cards, and which regions have surfaces which are of baremetal or which workably approximate that condition. There are numerousother variants within the invention hereof of the embodiments abovedescribed.

Accordingly, the invention is not to be considered as limited save as isconsonant with the recitals of the following claims.

I claim:
 1. A method of adding electromagnetic interference ("EMI")reduction capability to a circuit card comprising an insulative boardand a plastic faceplate fastened to the front of such board so that therespective centerplanes of such board and faceplate are at right anglesto each other, and such faceplate having front and back sides, saidmethod comprising:providing on the back side of said faceplate a pair ofplastic studs spaced from each other in the length of said faceplate andintegral therewith and projecting rearward from said back side thereof,providing an elongated metallic constant thickness strip having frontand back sides separated by the thickness of the strip, and havingformed therein, laterally inwards of the ends thereof, a pair of holesspaced apart in the length of said strip by the same distance as thatseparating said studs and of diameters matching that of said studs, thefront and back surface portions of said strip bordering said holes beingparallel planar surface portions, flatly abutting the front side of saidstrip with the back side of said faceplate by passing said studs throughsaid holes so that said studs fit therein without play and portions ofsaid studs project outward of said strip, and upsetting said portions totransform them into enlarged heads holding said strip fast to and flatagainst said plate.